PHYSIOLOGY. 



Horizontal leaves and shoots can be shown to be subject to 

 the same influence, and are therefore diageotropic. 



198. Influence of light. Not only is light a very important 

 factor for plants during carbon conversion, it exerts great influ- 

 ence on plant growth and movement. 



199. Retarding influence of light on growth. We have 

 only to return to the experiments performed in growing plants in 

 the dark to see 



one of the influ- 

 ences which light 

 exerts on plants. 

 The plants grown 

 in the dark were 

 longer and more 

 slender than those 

 grown in the light. 

 Light then has a 

 retarding influ- 

 ence on the elong- 

 ation of the stem. 



200. Influence 



Fig. 7 



f -i - -U j. j Radish seedlings grown in the 



01 llgnt On QireC- dark, long, slender, not green. 



tion of growth. While we are growing 



seedlings, the pots or boxes of some of them 



should be 'placed so that the plants will Fi 



have a one-sided illumination. This can Radish seedlings grown in 



the light, shorter, stouter, and 



be done by placing them near an open green in color. Growth re- 



. , . b . , . , , . tarded by light. 



window, in a room with a one-sided illu- 

 mination, or they may be placed in a box closed on all sides 

 but one which is facing the window or light. In 12-24 hours, 

 or even in a much shorter time in some cases, the stems of the 

 seedlings will be directed toward the source of light. This 

 influence exerted by the rays of light is heliotropism, a turning 

 influenced by the sun or sunlight. 



201. Diaheliotropism. Horizontal leaves and shoots are 

 diaheliotropic as well as diageotropic. The general direction 



